1. Field of the Invention
The present invention generally relates to an optical recording medium system, and more particularly, to a method and apparatus for controlling reproduction of an optical recording medium that generates a servo control signal by detecting the direction of an object lens in a high-density optical recording medium.
2. Background of the Related Art
Generally, an optical disc recording/reproducing apparatus is an apparatus for recording/reproducing data on/from an optical recording medium, i.e., optical disc such as a compact disc (CD), digital versatile disc (DVD), etc.
At this time, as rewritable digital versatile discs, there exist a rewritable DVD (DVD-RW), recordable DVD (DVD-R), DVD-RAM, etc.
According to the DVD-RAM, data is written on both a land and a groove, and according to the DVD-RW/R, data is written on the groove only. That is, according to the DVD-RW/R, positional information is written on a groove track by pre-pitting a land track instead of a header region of the DVD-RAM that represents the positional information, but the data is not written on the land track. In other words, information on a physical address of the groove track is written in advance on the land in the form of a pit.
FIG. 1 is a block diagram illustrating the construction of a general optical disc recording/reproducing apparatus for recording and reproducing data in the above-described DVD series optical discs. Referring to FIG. 1, an optical pickup 102, under the control of a servo control section 104, puts an optical beam condensed by an object lens on a signal track of an optical disc. The optical beam reflected from a signal recording surface of the optical disc is condensed through the object lens, and then incident to an optical detector for detection of a focus error signal and a tracking error signal.
The optical detector is composed of a plurality of optical detection elements, and electric signals in proportion to light quantities obtained by the respective optical detection elements are outputted to an RF and servo error generating section 103.
The RF and servo error generating section 103 detects an RF signal for data reproduction, focus error (FE) signal for servo control, tracking error (TE) signal, etc., from the electric signals outputted from the respective optical detection elements of the optical detector.
The detected RE signal is outputted to a data decoder for reproduction, and the servo error signals such as the FE and TE signals are outputted to the servo control section 104.
The servo control section 104 processes the focus error (FE) signal to output a driving signal for focus control to a focus servo driving section 105, and processes the tracking error (TE) signal to output a driving signal for tracking control to a tracking servo driving section 106.
Then, the focus servo driving section 105 moves the optical pickup 102 up and down by driving a focus actuator in the optical pickup 102, so that the optical pickup 102 follows the up/down movement of the rotating optical disc 101.
The tracking servo driving section 106 moves the object lens of the optical pickup 102 in a radial direction by driving a tracking actuator in the optical pickup 102, and thus corrects the position of the beam to follow a specified track.
The RF and servo error generating section 103 and servo control section 104 generally use various kinds of tracking control methods such as a three-beam method, push-pull (PP) method, differential phase detection (DPD) method, etc., for the tracking control in the DVD series optical discs.
According to the push-pull (PP) method, the optical detection elements of the optical detector for detecting the optical beam reflected from the optical disc are divided into two parts in a track direction, and the tracking error signal is detected from a light quantity balance of the two-divided optical detection elements. Specifically, this method uses the fact that the intensity distribution of light, that is diffracted and reflected by a pit and then incident again to the object lens, varies according to the relative positional change of the pit and the spot.
At this time, if the shadow of the pit is equally detected by the both optical detection elements, the tracking error (TE) signal becomes “0”, and this state is called a tracking-on (or on-track) state. On the contrary, if the optical beam deviates left or right from the track center, the tracking error (TE) signal has a positive (+) or negative (−) value, and this state is called a tracking-off (or off-track) state.
The PP method has several conditions. According to one among them, if the wavelength of the light is λ and the depth of the pit is λ/4, i.e., if the diffraction by the pit is most effective and the depth of modulation becomes maximum, the tracking error signal cannot be obtained through the PP method. In other words, since the incident light and the reflected light are offset due to an interference therebetween if the depth of the pit is λ/4, the tracking error signal cannot be obtained through the 2-divided optical detector.
Meanwhile, the DPD method is an improvement of the PP method. In the same manner as the PP method, the DPD method uses the intensity distribution of light according to the relative positional change of the beam and pit, but it uses a 4-divided optical detector instead of the 2-divided optical detector.
Specifically, according to the DPD method, the intensity distribution of light is received through the 4-divided optical detector, and the tracking error signal is generated through the detection of phase difference in the radial direction.
Accordingly, the tracking error signal is outputted even if the depth of the pit is λ/4, and is not much affected by the movement of the beam on the optical detector as well.
For example, as shown in FIG. 2, in case that the optical detector is composed of 4 optical detection elements PDA, PDB, PDC, and PDD divided in the signal track direction and radial direction of the optical disc, the optical detector outputs electric signals a, b, c, and d in proportion to the light quantities obtained by the respective optical detection elements PDA, PDB, PDC, and PDD.
At this time, the DPD method obtains the tracking error (TE) signal through the detection of the phase difference between diagonal difference signals, i.e., between the electric signal of “a+c” and the electric signal of “b+d” at a slice point of the RF signal of “a+b+c+d” obtained from the electric signals a, b, c, and d outputted from the optical detector. That is, by detecting the phase difference, the positive TE signal can be obtained. As described above, the TE signal according to the DPD method is generated using the phase difference in the radial direction detected while the object lens passes the pit on the track.
As shown in FIG. 3(b), if the track is in the center of the beam, the DPD signal becomes zero, and this value is maintained even if the beam moves in a direction as indicated as an arrow by the rotation of the disc. This means that the phase difference signal in the diagonal direction is not generated.
Meanwhile, as shown in FIGS. 3(a) and 3(c), if the beam deviates from the track and moves in a direction as indicated as an arrow, the DPD signal becomes an output of sine wave. The phase of the sine wave deviates by ±90° with respect to that of the RF signal. Thus, by detecting the phase of the DPD signal at the slice point of the RF signal based on the RF signal, the positive and negative tracking error signals can be obtained.
Hereinafter, the tracking error signal obtained through the PP method is referred to as a PP signal, and the tracking error signal obtained through the DPD method is referred to as a DPD signal.
Also, in case that the optical detector is divided into two in the track direction, the tracking error signal is detected from the light quantity balance of both photodiodes I1 and I2 through the PP method. That is, the electric signals a and d correspond to the photodiode I1, and the electric signals b and c correspond to the photodiode I2.
At this time, the DVD-ROM generates the tracking error signal using the DPD method. Specifically, since the depth of the pit is λ/4 in case of the DVD-ROM, the tracking error signal cannot be detected through the PP method. Thus, the DVD-ROM obtains the tracking error signal using the DPD method. Also, the DVD-R or DVD-RW detects the tracking error signal using the DPD method in case of reproducing a region where the signal is recorded, while it detects the tracking error signal using the PP method in case of recording the signal. Also, the DVD-RAM detects the tracking error signal using the DPD method only with respect to a pre-pit region, while it detects the tracking error signal using the PP method with respect to other regions.
Accordingly, in controlling the above-described DVD-series optical disc, it is very important to detect the land and the groove.
For example, in case of the DVD-RW/R where data has already been written, a mirror signal is generated using the difference of radial contrast (RC), and the track (i.e., pit or groove) and the mirror (i.e. land) are discriminated using the mirror signal.
This is because it is the best time point for the track-on when the object lens of the tracking actuator is in a direction of an outer periphery and the head of the optical pickup is in the center of the groove simultaneously.
However, if the RC is too small to be detected, i.e., if it is difficult to secure the RC due to the high-density and high-speed operation and the mirror is not properly detected as well, it becomes difficult to perform the land/groove discrimination and also an automatic breaking system does not operate. This causes the servo to be unstable during the track-on.